Spark plug

ABSTRACT

A spark plug comprising: a center electrode; a metal shell; and an alumina ceramic insulator disposed between the center electrode and the metal shell, wherein at least part of the surface of the insulator is covered with a glaze layer comprising oxides, wherein the glaze layer comprises: 1 mol % or less of a Pb component in terms of PbO; 30 to 60 mol % of a Si component in terms of SiO 2 ; 20 to 50 mol % of a B component in terms of B 2 O 3 ; 0.5 to 25 mol % of a Zn component in terms of ZnO; 0.5 to 15 mol % in total of at least one of Ba and Sr components in terms of BaO and SrO, respectively; 2 to 12 mol % in total of at least two alkaline metal components of Na, K and Li, in terms of Na 2 O, K 2 O, and Li 2 O, respectively, wherein K and Li is essential; and 0.1 to 10 mol % of a F component in terms of F 2 .

FIELD OF THE INVENTION

[0001] This invention relates to a spark plug.

BACKGROUND OF THE INVENTION

[0002] A spark plug used for ignition of an internal engine of such asautomobiles generally comprises a metal shell to which a groundelectrode is fixed, an insulator made of alumina ceramics, and a centerelectrode which is disposed inside the insulator. The insulator projectsfrom the rear opening of the metal shell in the axial direction. Aterminal metal fixture is inserted into the projecting part of theinsulator and is interconnected to the center electrode via a conductiveglass seal layer which is formed by a glass sealing procedure or aresistor. A high voltage is applied to the terminal metal fixture tocause a spark over the gap between the ground electrode and the centerelectrode.

[0003] Under some combined conditions, for example, at an increasedspark plug temperature and an increased environmental humidity, it mayhappen that high voltage application fails to cause a spark over the gapbut, instead, a discharged called as a flashover occurs between theterminal metal fixture and the metal shell, going around the projectinginsulator. Primarily for the purpose of avoiding flashover, most ofcommonly used spark plugs have a glaze layer on the surface of theinsulator. The glaze layer also serves to smoothen the insulator surfacethereby preventing contamination and to enhance the chemical ormechanical strength of the insulator.

[0004] In the case of the alumina insulator for the spark plug, such aglaze of lead silicate glass has conventionally been used where silicateglass is mixed with a relatively large amount of PbO to lower adilatometric softening point. In recent years, however, with a globallyincreasing concern about environmental conservation, glazes containingPb have been losing acceptance. In the automobile industry, forinstance, where spark plugs find a huge demand, it has been a subject ofstudy to phase out Pb glazes in a future, taking into consideration theadverse influences of wasted spark plugs on the environment.

[0005] Leadless borosilicate glass- or alkaline borosilicate glass-basedglazes have been studied as substitutes for the conventional Pb glazes,but they inevitably have inconveniences such as a high glass viscosityor an insufficient insulation resistance. In particular, in the case ofthe glaze for spark plugs, since being served together with engines, itmore easily increases temperature than ordinary insulating porcelains(maximum: around 200° C.), and recently being accompanies with highperformance of engines, voltage to be supplied to the spark plug hasbeen high, and the glaze has been demanded to have the insulatingperformance durable against more severer. Actually, for restraining theflashover under a condition of increasing temperature, such a glaze isnecessary which is more excellent in the insulating property under thecondition of increasing temperature.

SUMMARY OF THE INVENTION

[0006] In the existing leadless glaze for spark plugs, for checking amelting point from going up effected by removing a lead component, analkaline metal component has been mixed. The alkaline metal component iseffective for securing fluidity when baking the glaze. However, the morethe content of the alkaline metal component, the lower the insulatingresistance of the glaze, and an anti-flashover property is easilyspoiled. Therefore, the alkaline metal component in the glaze should belimited to a necessary minimum for increasing the insulating property.

[0007] So, the existing leadless glaze has inevitably wanted the contentof the alkaline metal, a vitreous viscosity is likely to increase athigh temperature (when melting the glaze) in comparison with a Pb-glaze,and after baking the glaze, there easily appear pinholes or glazecrimping. For removing these defects, it is assumed to heighten theglaze baking temperature so as to improve the fluidity, but theheightening of the glaze baking temperature is not preferable since itinvites an energy cost-up and to shorten lives of facilities.

[0008] It is an object of the invention to offer such a spark plug whichcontains a smaller Pb component, is excellent in the fluidity whenbaking the glaze, high in the insulating resistance, and good in theanti-flashover.

[0009] The spark plug according to the invention has a structure havingan alumina ceramic insulator disposed between a center electrode and ametal shell, wherein at least part of the surface of the insulator iscovered with a glaze layer of oxide being a main.

[0010] In this first structure, the glaze layer is characterized bycomprising

[0011] Pb component 1 mol % or less in terms of PbO;

[0012] Si component 30 to 60 mol % in terms of SiO₂;

[0013] B component 20 to 50 mol % in terms of B₂O₃;

[0014] Zn component 0.5 to 25 mol % in terms of ZnO;

[0015] Ba and/or Sr components 0.5 to 15 mol % in terms of BaO or SrO intotal;

[0016] alkaline metal components of 2 to 12 mol % in total of

[0017] two kinds or more of Na in terms Na₂O, K in terms of K₂O and Liin terms of Li₂O, K and Li being essential, respectively; and

[0018] F component 0.1 to 10 mol % in terms of F₂.

[0019] In a second structure, the glaze layer is characterized bycomprising

[0020] Pb component 1 mol % or less in terms of PbO;

[0021] Si component 30 to 60 mol % in terms of SiO₂;

[0022] B component 20 to 40 mol % in terms of B₂O₃;

[0023] Zn component 0.5 to 25 mol % in terms of ZnO;

[0024] Ba and/or Sr components 0.5 to 15 mol % in terms of BaO or SrO intotal;

[0025] alkaline metal components of 2 to 12 mol % in total of

[0026] one kind or more of Na in terms Na₂O, K in terms of K₂O and Li interms of Li₂O, respectively;

[0027] F component 0.1 to 10 mol % in terms of F₂; and

[0028] one kind or more selected from Bi, Sb and rare earth elements RE(selected from a group of Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho,Er, Tm, Yb and Lu) of 0.1 to 5 mol % in total of Bi in terms of Bi₂O₃,Sb in terms of Sb₂O₅, as to RE, Ce in terms of CeO₂, Pr in terms ofPr₇O₁₁, and others in terms of RE₂O₃.

[0029] In the spark plug according to the invention, for aiming at theadaptability to the environmental problems, it is a premise that theglaze to be used contains the Pb component 1.0 mol % or less in terms ofPbO (hereafter called the glaze containing the Pb component reduced tothis level as “leadless glaze”). When the Pb component is present in theglaze layer in the form of an ion of lower valency (e.g., Pb²⁺), it isoxidized to an ion of higher valency (e.g., Pb³⁺) by a corona discharge.If this happens, the insulating properties of the glaze layer arereduced, which probably spoils an anti-flashover. From this viewpoint,too, the limited Pb content as mentioned above is beneficial. Apreferred Pb content is 0.1 mol % or less. It is most preferred for theglaze to contain substantially no Pb (except a trace amount of leadunavoidably incorporated from raw materials of the glaze).

[0030] While lowering the Pb content as mentioned above, the inventionselects the above mentioned particular compositions for providing theinsulating performance, optimizing the glaze baking temperature(actually, lowering temperature) and securing a good glaze-baked finish.In the existing glaze, the Pb component plays an important part as toadjustment of the dilatometric softening point (practically,appropriately lowering the dilatometric softening point of the glaze andsecuring the fluidity when baking the glaze) but in the leadless glaze,the B component (B₂O₃) and the alkaline metal have a deep relation withadjustment of the dilatometric softening point. Inventors found that theB component has a particularly convenient range for improving the glazebaking finish in relation with the content of the Si component, and ifthe F component is contained in the above mentioned range, the fluiditywhen baking the glaze may be secured while controlling the content ofthe alkaline metal to be relatively low, and in turn the baking of theglaze is possible at relatively low temperatures, the glaze layer havingan excellent and smooth baked surface is available, and they completedthis invention.

[0031] Detailed explanation will be made to roles and criticalsignificances of the respective components (the explanation is common tothe first and second structures, excepting especial remarks).

[0032] The alkaline metal component is inherently high in ionconductivity and trends to lower the insulating property in the glazelayer of vitreous substance. On the other hand, the Si component or theB component form a vitreous skeleton, and by appropriately determiningthe contents, sizes of network of skeleton are made suitable forblocking the ion conductivity of the alkaline metal and securing thedesirable insulating property. Since the Si component or the B componentare ready for forming skeleton, they trend to lower the fluidity whenbaking the glaze, but by containing the alkaline metal component of theappropriate amount together with the components of improving fluidity,the fluidity is heightened by lowering melting points by a eutecticreaction and preventing formation of complex anion by mutual action ofSi ion and 0 ion.

[0033] The Si component is difficult to secure the sufficient insulatingproperty if being less than 30 mol %, and is difficult to bake the glazeif being more than 60 mol %. On the other hand, if the B component isless than 20 mol %, the dilatometric softening point of the glaze risesand the baking of the glaze is difficult. An upper limit of the Bcomponent is 50 mol % in the first structure and 40 mol % in the secondstructure. If the B component is contained over these upper limits, thefluidity exceedingly increases, crimping is easily created in the glaze.In the second structure, the increasing of the fluidity is prospected byan amount of containing a fluidity improving component though the Bcomponent is lower than that of the first structure. Accordingly, theupper limit of the B component is determined to be lower than that ofthe first structure in response to the minimum addition amount (0.1 mol%) of the fluidity improving component. If the B content exceeds theupper limits, depending on contents of other components, there probablyoccur problems about devitrification of the glaze layer, decrease of theinsulating property or non-compatibility with thermal expansioncoefficient.

[0034] If the Zn component is less than 0.5 mol %, the thermal expansioncoefficient of the glaze layer is too large, defects such as crazingeasily occur in the glaze layer. Since the Zn component also acts tolower the dilatometric softening point of the glaze, if it is short, thebaking of the glaze will be difficult. Being more than 25 mol %, opacityeasily occurs in the glaze layer due to the devitrification. It is goodthat the Zn containing amount to determine 10 to 20 mol %. Whencontaining the Zn component within this desirable range, the fluidityimproving effect can be also expected by lowering of the dilatometricsoftening point of the Zn component itself, and in this case, the totalamount of the fluidity improving components is desirably 0.1 to 2.5 mol%.

[0035] The Ba or Sr components contribute to heightening of theinsulating property of the glaze layer and is effective to increasing ofthe strength. If the total amount is less than 0.5 mol %, the insulatingproperty of the glaze layer goes down, and the anti-flashover might bespoiled. Being more than 15 mol %, the thermal expansion coefficient ofthe glaze layer is too high, defects such as crazing easily occur in theglaze layer. In addition, the opacity easily occurs in the glaze layer.From the viewpoint of heightening the insulating property and adjustingthe thermal expansion coefficient, the total amount of Ba and Sr isdesirably determined to be 0.5 to 10 mol %. Either or both of the Ba andSr component may be contained, but the Ba component is advantageouslyless expensive in a cost of a raw material.

[0036] The Ba and Sr components may exist in forms other than oxides inthe glaze depending on raw materials to be used. For example, BaSO₄ isused as a source of the Ba component, an S component might be residualin the glaze layer. This sulfur component is concentrated nearly to thesurface of the glaze layer when baking the glaze to lower the surfaceexpansion of a melted glaze and to heighten a smoothness of a glazelayer to be obtained.

[0037] The total amount of the Zn component and Ba and/or Sr componentsis desirably 7 to 25 mol % in terms of oxide. If the total amountexceeds 25 mol %, the glaze layer will be slightly opaque. For example,on the outer surface of the insulator, visual information such asletters, figures or product numbers are printed and baked with colorglazes for identifying makers and others, and owing to the slightopaqueness, the printed visual information is sometimes illegible. Or,if being less than 7 mol %, the dilatometric softening point exceedinglygoes up to make the glaze baking difficult and cause bad externalappearance. Thus, the total amount is more desirably 10 to 20 mol %.

[0038] Next, if the total amount of the alkaline metal components

[0039] is less than 2 mol %, the dilatometric softening point of theglaze goes up, and the baking of the glaze might be probably impossible.In case of being more than 12 mol %, the insulating property probablygoes down, and an anti-flashover might be spoiled. With respect to thealkaline metal components, not depending on one kind, but adding injoint two kinds or more selected from Na, K and Li, the insulatingproperty of the glaze layer is more effectively restrained fromlowering. As a result, the amount of the alkaline metal components canbe increased without decreasing the insulating property, consequently itis possible to concurrently attain the two purposes of securing thefluidity when baking the glaze and the anti-flashover (so-calledalkaline joint addition effect).

[0040] In the first structure, as to the alkaline metal components, Kand Li are indispensably contained. As the K component has a largeratomic amount than those of Na and Li, in case the total amount of thealkaline metal components is set to be the same mol %, the K componentdoes not exhibit the fluidity improving effect as the Na or Licomponents, but comparing with Na or Li (particularly, Li), as an ionicmigration of K is comparatively small in the glaze layer of the vitreoussubstance, the K component has an inclination difficult to lower theinsulating property of the glaze layer, though increasing the amount. Onthe other hand, as the Li component has the small atomic amount, thefluidity improving effect is larger than that of the K component, but asthe ionic migration is high, an exceeding addition easily brings aboutreduction of the insulating property of the glaze layer.

[0041] Therefore, in the first structure, for always securing thefluidity of a necessarily enough level also in case a later mentionedfluidity improving component is not added, an inclusion of the Licomponent having a large fluidity improving effect is indispensable, andfor compensating reduction of the insulating property by increase of theLi component, an addition of K is made a premise. Of course, at leasttwo kinds of alkaline metal components are added in joint, so that theinsulating property improving effect by the joint addition of alkalinemetals is accomplished (on the other hand, in the second structure basedon the premise of adding the fluidity improving component, no limitationis made to kinds of the alkaline metal components to be contained.)

[0042] For example, among the alkaline metal components, it is possibleto effectively restrain the insulating property of the glaze layer fromlowering by making the amount of the K component highest, and by mixingthe Li component of the amount next to the highest amount of K, it ispossible to secure the fluidity when baking the glaze, restrain increaseof the thermal expansion coefficient of the glaze layer by mixing the Kcomponent, and match with the thermal expansion coefficient of aluminain a substrate. The inclination of the insulating property decreasing byaddition of the Li component can be effectively restrained by the abovementioned joint addition of alkaline metals by the three components bycompounding Na of the smaller amount than those of K or Li. As a result,it is possible to realize such a glaze composition which is high in theinsulating property, rich in the fluidity when baking the glaze, andsmall in difference between the thermal expansion coefficients with thatof alumina being the insulator composing ceramic.

[0043] Specifically, it is desirable to set the rate of the K componentof the alkaline metal components of Na, K and Li in the mol % in termsof oxide as

0.4≦K/(Na+K+Li)≦0.8.

[0044] If the value of K/(Na+K+Li) is less than 0.4, the insulatingproperty improving effect by the K addition might be insufficient. Onthe other hand, that the value of K/(Na+K+Li) is less 0.8 denotes thatalkaline metal components other than K are added in joint within a rangeof a rest being 0.2 or more (0.6 or less), and it is possible toheighten the insulating property by the above mentioned joint additionof alkaline and in turn to improve the anti-flashover. Incidentally, itis desirable to adjust the value of K/(Na+K+Li) to be 0.5 to 0.7.

[0045] The Li component is preferred to be contained in order to realizethe effect of adding in joint alkaline components for increasing theinsulating property, and in order to adjust the heat expansioncoefficient of the glaze layer, to secure the fluidity when baking theglaze, and further to increase the mechanical strength. It is preferablethat the Li component is contained in the mol amount in terms of oxidein the following range:

0.2≦Li/(Na+K+Li)>0.5.

[0046] If the rate of Li is less than 0.2, the heat expansioncoefficient becomes too large in comparison with the alumina substrate.As a result, the crazing maybe easily produced to make the baked surfacefinish of the glaze insufficient. On the other hand, if the rate of Licomponent exceeds 0.5, this may give an adverse influence to theinsulating property of the glaze layer because the Li ion has acomparatively high degree of immigration among the alkaline metal ions.It is preferable that the value of Li/(Na+K+Li) is adjusted in the rangeof 0.3 to 0.45.

[0047] Next, if the F component adds together with the alkaline metalcomponents, it exhibits effects of lowering the dilatometric softeningpoint of the glaze and improving the fluidity when baking the glaze,though controlling the content of the alkaline metal component to below. If the content is less than 0.1 mol % in terms of F₂, the fluidityimproving effect is insufficient, and if being more than 10 mol %, airbubbles are ready for arising which are likely to cause breakdown in theglaze when baking it, and this attributes to spoiling of the strength ofthe insulator having the glaze layer thereon, for example, the impactresistance, and the glaze layer is likely to devitrify owing to muchbubbles. Further, a gas containing the F component is issued when bakingthe glaze, and this trends to invite inconveniences of reacting with arefractory composing an oven wall to shorten the life of the oven wall.The F component is preferably contained 2 to 6 mol % in terms of F₂.

[0048] Further, it is desirable to adjust the fluidity improving effectby the F addition in response to the addition amount of the alkalinemetal components. Specifically, if the total mol containing rate (mol %)in terms of oxide of the alkaline metal components is NR (mol %) and themol containing rate of the F component in terms of F2 is NF, preferablyNF/NR is 0.07 to 1.5. If being less than 0.07, the fluidity improvingeffect by the F addition is insufficient, and if being more than 1.5, aremarkable heightening of the fluidity improving effect by increasingthe F addition is not prospective, and futility is much.

[0049] By the way, the F component can be added by compounding a part ofa source of a cation component of the glaze layer in a form of fluorideof this cation, for example, in the form of fluoride of Si, alkalinemetal, alkaline earth metals, or rare earth metals (actually, LiF orCaF₂, provided that the containing rates of the cation components addedin the form of fluoride are shown in terms of oxides in this invention).As the fluoride of silicon, for example, silicon fluoride based highpolymer can be employed. F compounds dissolved or exhausted in forms ofgas of components other than F when preparing the glaze frit, can beadded, for example, in a form of fluoride of carbon(polytetrafluoroethylene or graphite fluoride).

[0050] Next, in the second structure, the above mentioned fluidityimproving components are indispensably contained. Each of these fluidityimproving components has effects of heightening the fluidity when bakingthe glaze, controlling the bubble forming in the glaze layer, orwrapping adhered substances to the glaze baked surface to preventabnormal projections. Sb and Bi are especially remarkable in theseeffects (Bi has possibility to be designated as a limited substance in afuture). The improvement of the fluidity when baking the glaze is moreremarkable by combining two kinds or more of these fluidity improvingcomponents. Since the rare earth component comparatively takes cost forseparation and refinement, use of non-separating rare earth elements (inthis case, those are the composition particular to raw ores and aplurality of kinds of rare earth elements are mixed) is advantageous forsaving cost. If the total amount in terms of oxides of the indispensablefluidity improving components is less than 0.1 mol %, there will beprobably a case of not always providing an effect of improving thefluidity when baking the glaze for easily obtaining a smooth glazelayer. On the other hand, if exceeding 5 mol %, there will be probably acase of being difficult or impossible to bake the glaze owing to toomuch heightening of the softening point of the glaze.

[0051] If parts of Sb, Bi and the rare earth components are more than 5mol % in the addition amount, the glaze layer might be excessivelycolored. For example, visible information such as letters, figures orproduct numbers are printed with color glazes on external appearances ofthe insulators for specifying producers and others, and if the colors ofthe glaze layer is too thick, it might be difficult to read out theprinted visible information. As another realistic problem, there is acase that tint changing resulted from alternation in the glazecomposition is seen to purchasers as “unreasonable alternation infamiliar colors in external appearance”, so that an inconvenience occursthat products could not always be quickly accepted because of aresistant feeling thereto.

[0052] The insulator forming a substrate of the glaze layer is composedof alumina based ceramics in white, and in view of preventing orrestraining coloration, it is desirable that the coloration in anobserved external appearance of the glaze layer formed in the insulatoris adjusted to be 0 to 6 in chroma Cs and 7.5 to 10 in lightness Vs, forexample, the amount of the above transition metal component is adjusted.If the chroma exceeds 6, discrimination by naked eye is conspicuous, andif lightness is 7.5 or lower, the gray or blackish coloration is easilydistinguished. In either way, there appears a problem that an impressionof “apparent coloration” cannot be wiped out. The chroma Cs is desirably0 to 2, more desirably 0 to 1, and the chroma is preferably 8 to 10,more preferably 9 to 10. In the present specification, a measuringmethod of the lightness Vs and the chroma Cs adopts the method specifiedin “4.3 A Measuring Method of Reflected Objects” of “4. SpectralColorimetry” in the “A Measuring Method of Colors” of JIS-Z8721. As asimple method, the lightness and the chroma can be known through visualcomparisons with standard color chart prepared according to JIS-Z8721.

[0053] In the following description, explanation will be made to othercomponents which can be contained in the glaze layer. At first, asauxiliary fluidity improving components, one kind or more of Mo, W, Ni,Co, Fe and Mn are contained 0.5 to 5 mol % in total in terms of MoO₃,WO₃, Ni₃O₄, Co₃O₄, Fe₂O₃ and MnO₂, respectively. If being less than 0.5mol %, an effect is insufficient, while being more than 5 mol %, thedilatometric softening point of the glaze exceedingly goes up, and theglaze-baking is difficult or impossible. Among the auxiliary fluidityimproving components, the most remarkable fluidity improving effects areMo and Fe, and next is W.

[0054] As each of these auxiliary fluidizing improving components istransition element, an excessive addition contributes to inconvenienceof causing unintentional coloring in the glaze layer (this might be aproblem when using the rare earth element as the fluidity improvingcomponent).

[0055] It is possible to contain one kind or more of Ti, Zr and Hf 0.5to 5 mol % in total in terms of ZrO₂, TiO₂ and HfO₂.

[0056] By containing one kind or more of Ti, Zr or Hf, a waterresistance is improved. As to the Zr or Hf components, the improvedeffect of the water resistance of the glaze layer is more. noticeable.By the way, “the water resistance is good” is meant that if, forexample, a powder like raw material of the glaze is mixed together witha solvent as water and is left as a glaze slurry for a long time, suchinconvenience is difficult to occur as increasing a viscosity of theglaze slurry owing to elusion of the component. As a result, in case ofcoating the glaze slurry to the insulator, optimization of a coatingthickness is easy and unevenness in thickness is reduced. Subsequently,said optimization and said reduction can be effectively attained. Ifbeing less than 0.5 mol %, the effect is poor, and if being more than 5mol %, the glaze layer is ready for devitrification.

[0057] It is possible to contain 0.5 to 15 mol % in total of one kind ormore of the Al component 0.5 to 5 mol % in terms of Al₂O₃, the Cacomponent 0.5 to 10 mol % in terms of CaO, and the Mg component 0.5 to10 mol % in terms of MgO. The Al component has an effect of restrainingthe devitrification of the glaze layer, the Ca component and the Mgcomponent contribute to improvement of the insulating property of theglaze layer. In particular, the Ca component is effective next to the Bacomponent or the Zn component for increasing the insulating property ofthe glaze layer. If the addition amount is less than each of the abovementioned lower limits, the effect is insufficient, while being morethan the upper limit of each of the components or the upper limit of thetotal amount, the dilatometric softening point exceedingly increases andthe glaze-baking might be difficult or impossible.

[0058] The glaze layer may contain auxiliary components of one kind ormore of Sn, P, Cu, and Cr 0.5 to 5 mol % in total as Sn in terms ofSnO₂, P in terms of P₂O₅, Cu in terms of CuO, and Cr in terms of Cr₂O₃.These components may be positively added in response to purposes oroften inevitably included as raw materials of the glaze (otherwise latermentioned clay minerals to be mixed when preparing the glaze slurry) orimpurities (otherwise contaminants) from refractory materials in themelting procedure for producing glaze frit. Each of them heightens thefluidity when baking the glaze, restrains bubble formation in the glazelayer, or wraps adhered materials on the baked glaze surface so as toprevent abnormal projections. If the addition amount is less than eachof the above mentioned lower limits, the effect is insufficient, whilebeing more than the upper limit of each of the components or the upperlimit of the total amount, the dilatometric softening point exceedinglyincreases and the glaze-baking might be difficult or impossible (inparticular, CuO and Cr₂O₃), or insufficient conductivity (in particular,by excessive amount of SnO₂) or insufficient water resistance (inparticular, by excessive amount of P₂O₅) of the glaze layer is caused.

[0059] In the structure of the spark plug of the invention, therespective components in the glaze are contained in the forms of oxides,and owing to factors forming amorphous and vitreous phases, the existingforms by oxides cannot be often identified. In this case, if thecontaining amounts of components at values in terms of oxides in theglaze layer fall in the above mentioned ranges, it is regarded that theybelong to the ranges of the invention.

[0060] Herein, the containing amounts of the respective components inthe glaze layer formed on the insulator can be identified by use ofknown micro-analyzing methods such as EPMA (electronic probemicro-analysis) or XPS (X-ray photoelectron spectroscopy). For example,if using EPMA, either of a wavelength dispersion system and an energydispersion system is sufficient for measuring characteristic X-ray.Further, there is a method where the glaze layer is peeled from theinsulator and is subjected to a chemical analysis or a gas analysis foridentifying the composition.

[0061] The spark plug having the glaze layer of the invention may becomposed by furnishing, in a through hole of the insulator, a pole-liketerminal metal fixture as one body with the center electrode or byholding a conductive bonding layer in relation therewith, said metalfixture being separate from a center electrode. In this case, theinsulating resistant value can be measured under a condition where anelectric conductivity is made between the terminal metal fixture and ametal shell, keeping the whole of the spark plug at around 500° C. Forsecuring an insulating endurance at high temperatures, it is desirablethat the insulating resistant value is secured 200 MΩ or higher,desirably 400 MΩ so as to prevent the flashover.

[0062] The measurement may be carried out as follows. DC constantvoltage source (e.g., source voltage 1000 V) is interconnected to theside of a terminal metal 13 of the spark plug 100 shown in FIG. 1, whileat the same time, the side of the metal shell 1 is grounded, and acurrent is passed under a condition where the spark plug 100 disposed ina heating oven is heated at 500° C. For example, assuming that a currentvalue Im is measured by use of a current measuring resistance(resistance value Rm) at the voltage VS, an insulation resistance valueRx to be measured can be obtained as (VS/Im)−Rm.

[0063] The insulator may be composed of the alumina insulating materialcontaining the Al component 85 to 98 mol % in terms of Al₂O₃.Preferably, the glaze layer has an average thermal expansion coefficientof 5×10⁻⁶/° C. to 8.5×10⁻⁶/° C. at the temperature ranging 20 to 350° C.Being less than this lower limit of the average thermal expansion,defects such as cracking or graze skipping easily happen in the grazelayer. On the other hand, being more than the upper limit, defects suchas crazing are likely to happen in the graze layer. The thermalexpansion coefficient more preferably ranges 6×10⁻⁶/° C. to 8×10⁻⁶/° C.

[0064] The thermal expansion coefficient of the glaze layer is assumedin such ways that samples are cut out from a vitreous glaze bulk bodyprepared by mixing and melting raw materials such that almost the samecomposition as the glaze layer is realized, and values measured by aknown dilatometer method.

[0065] The thermal expansion coefficient of the glaze layer on theinsulator can be measured by use of, e.g., a laser inter-ferometer or aninteratomic force microscope.

[0066] The insulator may be formed with a projection radially extendingfrom the outer periphery at the middle portion in the axial directionthereof, and may be formed cylindrically in an outer periphery of thebase portion thereof adjacent the rear side with respect to theprojection thereof with a forward portion extending toward a forward endof the center electrode in the axial direction. In general, as toautomobile engines, a rubber cap is utilized to attach the spark plug tothe electric system of engines. In order to heighten the anti-flashover,adhesion between the insulator and the interior of the rubber cap isimportant. Therefore, the glaze layer desirably is smooth at a maximumheight of 7 μm or less in a curve of a surface roughness in accordanceto the measurement prescribed by JIS:B0601 at the outer periphery of thebase portion.

[0067] According to the study by the inventors, it was found that as toborosilicate glass based- or alkaline borosilicate glass based leadlessglaze layer, it was important to adjust the film thickness of the glazelayer for obtaining the smooth surface of the glaze layer. Further, itwas found that since the outer periphery in the base portion of theinsulator main part is required to closely contact the rubber cap, theadjustment of film thickness, if properly conducted, will increase theanti-flashover. In the insulator having the leadless glaze layer, it isdesirable to adjust the film thickness of the glaze layer covering theouter periphery in the base portion of the insulator main part withinthe range of 7 to 50 μm. Thus, the close contact may be obtained betweenthe glaze baked surface and the rubber cap without lowering theinsulating property of the glaze layer, and in turn the anti-flashovermay be obtained.

[0068] In case the thickness of the glaze layer in the insulator is lessthan 7 μm, it is difficult to form the uniform and smooth glaze bakedsurface in the leadless glaze layer of the above mentioned composition,and the close contact between the glaze baked surface and the rubber capis spoiled, so that the anti-flashover is made insufficient. On theother hand, in case the thickness of glaze layer exceeds 50 μm, a crosssectional area of conductivity increases, so that it is difficult tosecure the insulating property with the leadless glaze layer of thementioned composition, similarly, resulting in lowering of theanti-flashover.

[0069] For making the thickness of the glaze layer uniform andrestraining the glaze layer from excessive (or local) thickness, theaddition of Ti, Zr or Hf is useful as mentioned above.

[0070] The spark plug of the invention can be produced by a productionmethod comprising:

[0071] a step of preparing glaze powders in which the raw materialpowders are mixed at a predetermined ratio, the mixture is heated 1000to 1500° C. and melted, the melted material is rapidly cooled, vitrifiedand ground into powder;

[0072] a step of piling the glaze powder on the surface of an insulatorto form a glaze powder layer; and

[0073] a step of heating the insulator, thereby to bake the glaze powderlayer on the surface of the insulator.

[0074] The powdered raw material of each component includes not only anoxide thereof (sufficient with complex oxide) but also other inorganicmaterials such as hydroxide, carbonate, chloride, sulfate, nitrate, orphosphate. These inorganic materials should be those of capable of beingconverted to oxides by heating and melting. The rapidly cooling can becarried out by throwing the melt into a water or atomizing the melt ontothe surface of a cooling roll for obtaining flakes.

[0075] The glaze powder is dispersed into the water or solvent, so thatit can be used as a glaze slurry. For example, if coating the glazeslurry onto the insulator surface to dry it, the coating layer of theglaze powder (the glaze powder layer) can be formed. By the way, as themethod of coating the glaze slurry on the insulator surface, if adoptinga method of spraying from an atomizing nozzle onto the insulatorsurface, the glaze powder layer in uniform thickness of the glaze powdercan be easily formed and an adjustment of the coated thickness is easy.

[0076] The glaze slurry can contain an adequate amount of a clay mineralor an organic binder for heightening a shape retention of the glazepowder layer. As the clay mineral, those composed of mainlyaluminosolicate hydrates can be applied, for example, those composed ofmainly one kind or more of allophane, imogolite, hisingerite, smectite,kaolinite, halloysite, montmorillonite, illite, vermiculite, anddolomite (or mixtures thereof) can be used. In relation with the oxidecomponents, in addition to SiO₂ and Al₂O₃, those mainly containing onekind or more of Fe₂O₃, TiO₂, CaO, MgO, Na₂O and K₂O can be used.

[0077] The spark plug of the invention is constructed of an insulatorhaving a through hole formed in the axial direction thereof, a terminalmetal fixture fitted in one end of the through hole, and a centerelectrode fitted in the other end. The terminal metal fixture and thecenter electrode are electrically interconnected in the through hole viaan electrically conductive sintered body mainly comprising a mixture ofa glass and a conductive material (e.g., a conductive glass seal or aresistor). The spark plug having such a structure can be made by aprocess including the following steps.

[0078] An assembly step: a step of assembling a structure comprising theinsulator having the through hole, the terminal metal fixture fitted inone end of the through hole, the center electrode fitted in the otherend, and a filled layer formed between the terminal metal fixture andthe center electrode, which (filled layer) comprises the glass powderand the conductive material powder.

[0079] A glaze baking step: a step of heating the assembled structureformed with the glaze powder layer on the surface of the insulator attemperature ranging 800 to 950° C. to bake the glaze powder layer on thesurface of the insulator so as to form a glaze layer, and at the sametime softening the glass powder in the filled layer.

[0080] A pressing step: a step of bringing the center electrode and theterminal metal fixture relatively close within the through hole, therebypressing the filled layer between the center electrode and the terminalmetal fixture into the electrically conductive sintered body.

[0081] In this case, the terminal metal fixture and the center electrodeare electrically interconnected by the electrically conductive sinteredbody to concurrently seal the gap between the inside of the through holeand the terminal metal fixture and the center electrode. Therefore, theglaze baking step also serves as a glass sealing step. This process isefficient in that the glass sealing and the glaze baking are performedsimultaneously. Since the above mentioned glaze allows the bakingtemperature to be lower to 800 to 950° C., the center electrode and theterminal metal fixture hardly suffer from bad production owing tooxidation so that the yield of the spark plug is heightened. The bakingglaze step can be preceded to the glass sealing step.

[0082] The dilatometric softening point of the glaze layer is preferablyadjusted to range, e.g., 520 to 700° C. When the dilatometric softeningpoint is higher than 700° C., the baking temperature above 950° C. willbe required to carry out both baking and glass sealing, which mayaccelerate oxidation of the center electrode and the terminal metalfixture. When the dilatometric softening point is lower than 520° C.,the glaze baking temperature should be set lower than 800° C. In thiscase, the glass used in the conductive sintered body must have a lowdilatometric softening point in order to secure a satisfactory glassseal. As a result, when an accomplished spark plug is used for a longtime under a relatively high temperature environment, the glass in theconductive sintered body is liable to denaturalization, and where, forexample, the conductive sintered body comprises a resistor, thedenaturalization of the glass tends to result in deterioration of theperformance such as a life under load. Incidentally, the dilatometricsoftening point of the glaze is adjusted at temperature range of 520 to620° C.

[0083] The dilatometric softening point of the glaze layer is a valuemeasured by performing a differential thermal analysis on the glazelayer peeled off from the insulator and heated, and it is obtained as atemperature of a peak appearing next to a first endothermic peak (thatis, a second endothermic peak) which is indicative of a sag point. Thedilatometric softening point of the glaze layer formed in the surface ofthe insulator can be also estimated from a value obtained with a glasssample which is prepared by compounding raw materials so as to givesubstantially the same composition as the glaze layer under analysis,melting the composition and rapidly cooling.

BRIEF DESCRIPTION OF THE DRAWING

[0084] [FIG. 1]

[0085] A whole front and cross sectional view showing the spark plugaccording to the invention;

[0086] [FIG. 2]

[0087] A front view showing an external appearance of the insulatortogether with the glaze layer; and

[0088] [FIGS. 3A and 3B]

[0089] Vertical cross sectional views showing some examples of theinsulator.

DETAILED DESCRIPTION OF THE INVENTION

[0090] Modes for carrying out the invention will be explained withreference to the accompanying drawings showing embodiments. FIG. 1 showsan example of the spark plug of the first structure according to theinvention. The spark plug 100 has a cylindrical metal shell 1, aninsulator 2 fitted in the inside of the metal shell 1 with its tip 21projecting from the front end of the metal shell 1, a center electrode 3disposed inside the insulator 2 with its ignition part 31 formed at thetip thereof, and a ground electrode 4 with its one end welded to themetal shell 1 and the other end bent inward such that a side of this endmay face the tip of the center electrode 3. The ground electrode 4 hasan ignition part 32 which faces the ignition part 31 to make a spark gap% between the facing ignition parts 32.

[0091] The metal shell 1 is formed to be cylindrical of a metal such asa low carbon steel. It has a thread 7 therearound for screwing the sparkplug 100 into an engine block (not shown). Symbol le is a hexagonal nutportion over which a tool such as a spanner or wrench fits to fasten themetal shell 1.

[0092] The insulator 2 has a through hole 6 penetrating in the axialdirection. A terminal fixture 13 is fixed in one end of the through hole6, and the center electrode 3 is fixed in the other end. A resistor 15is disposed in the through hole 6 between the terminal metal fixture 13and the center electrode 3. The resistor 15 is interconnected at bothends thereof to the center electrode 3 and the terminal metal fixture 13via the conductive glass seal layers 16 and 17, respectively. Theresistor 15 and the conductive glass seal layers 16, 17 constitute theconductive sintered body. The resistor 15 is formed by heating andpressing a mixed powder of the glass powder and the conductive materialpowder (and, if desired, ceramic powder other than the glass) in a latermentioned glass sealing step. The resistor 15 may be omitted, and theterminal metal fixture 13 and the center electrode 3 may be integrallyconstituted by one seal layer of the conductive glass seal.

[0093] The insulator 2 has the through hole 6 in its axial direction forfitting the center electrode 3, and is formed as a whole with aninsulating material as follows. That is, the insulating material ismainly composed of an alumina ceramic sintered body having an Al contentof 85 to 98 mol % (preferably 90 to 98 mol %) in terms of Al2O3.

[0094] The specific components other than Al are exemplified as follows.

[0095] Si component: 1.50 to 5.00 mol % in terms of SiO₂;

[0096] Ca component: 1.20 to 4.00 mol % in terms of CaO;

[0097] Mg component: 0.05 to 0.17 mol % in terms of MgO;

[0098] Ba component: 0.15 to 0.50 mol % in terms of BaO; and

[0099] B component: 0.15 to 0.50 mol % in terms of B₂O₃.

[0100] The insulator 2 has a projection 2 e projecting outwardly, e.g.,flange-like on its periphery at the middle part in the axial direction,a rear portion 2 b whose outer diameter is smaller than the projectingportion 2 e, a first front portion 2 g in front of the projectingportion 2 e, whose outer diameter is smaller than the projecting portion2 e, and a second front portion 2 i in front of the first front portion2 g, whose outer diameter is smaller than the first front portion 2 g.The first front portion 2 g is almost cylindrical, while the secondfront portion 2 i is tapered toward the tip 21.

[0101] On the other hand, the center electrode 3 has a smaller diameterthan that of the resistor 15. The through hole 6 of the insulator 2 isdivided into a first portion 6 a (front portion) having a circular crosssection in which the center electrode 3 is fitted and a second portion 6b (rear portion) having a circular cross section with a larger diameterthan that of the first portion 6 a. The terminal metal fixture 13 andthe resistor 15 are disposed in the second portion 6 b, and the centerelectrode 3 is inserted in the first portion 6 a. The center electrode 3has an outward projection 3 c around its periphery near the rear endthereof, with which it is fixed to the electrode. A first portion 6 aand a second portion 6 b of the through hole 6 are interconnected eachother in the first front portion 2 g in FIG. 3A, and at the connectingpart, a projection receiving face 6 c is tapered or rounded forreceiving the projection 3 c for fixing the center electrode 3.

[0102] The first front portion 2 g and the second front portion 2 i ofthe insulator 2 connect at a connecting part 2 h, where a steppeddifference is formed on the outer surface of the insulator 2. The metalshell 1 has a projection 1 c on its inner wall at the position meetingthe connecting part 2 h so that the connecting part 2 h fits theprojection 1 c via a gasket ring 63 thereby to prevent slipping in theaxial direction. A gasket ring 62 is disposed between the inner wall ofthe metal shell 1 and the outer side of the insulator 2 at the rear ofthe flange-like projecting portion 2 e, and a gasket ring 60 is providedin the rear of the gasket ring 62. The space between the two gaskets 60and 62 is filled with a filler 61 such as talc. The insulator 2 isinserted into the metal shell 1 toward the front end thereof, and underthis condition, the rear opening edge of the metal shell 1 is pressedinward the gasket 60 to form a sealing lip 1 d, and the metal shell 1 issecured to the insulator 2.

[0103]FIGS. 3A and 3B show practical examples of the insulator 2. Thedimensions of these insulators are as follows.

[0104] Total length L1: 30 to 75 mm;

[0105] Length L2 of the first front portion 2g: 0 to 30 mm (exclusive ofthe connecting part 2 f to the projecting portion 2 e and inclusive ofthe connecting part 2 h to the second front portion 2 i);

[0106] Length L3 of the second front portion 2 i: 2 to 27 mm;

[0107] Outer diameter D1 of the main portion 2 b: 9 to 13 mm;

[0108] Outer diameter D2 of the projecting portion 2 e: 11 to 16 mm;

[0109] Outer diameter D3 of the first front portion 2 g: 5 to 11 mm;

[0110] Outer base diameter D4 of the second front portion 2 i: 3 to 8mm;

[0111] Outer tip diameter D5 of the second front portion 2 i (where theouter circumference at the tip is rounded or beveled, the outer diameteris measured at the base of the rounded or beveled part in a crosssection containing the center axial line O): 2.5 to 7 mm;

[0112] Inner diameter D6 of the second portion 6 b of the through hole6: 2 to 5 mm;

[0113] Inner diameter D7 of the first portion 6 a of the through hole 6:1 to 3.5 mm;

[0114] Thickness t1 of the first front portion 2 g: 0.5 to 4.5 mm;

[0115] Thickness t2 at the base of the second front portion 2 i (thethickness in the direction perpendicular to the center axial line O):0.3 to 3.5 mm;

[0116] Thickness t3 at the tip of the second front portion 2 i (thethickness in the direction perpendicular to the center axial line O;where the outer circumference at the tip is rounded or beveled, thethickness is measured at the base of the rounded or beveled part in across section containing the center axial line O): 0.2 to 3 mm; and

[0117] Average thickness tA ((t2+t3)/2) of the second front portion 2 i:0.25 to 3.25 mm.

[0118] In FIG. 1, a length LQ of the portion 2 k of the insulator 2which projects over the rear end of the metal shell 1, is 23 to 27 mm(e.g., about 25 mm).

[0119] The insulator 2 shown in FIG. 3A has the following dimensions.L1=about 60 mm, L2=about 10 mm, L3 =about 14 mm, D1=about 11 mm,D2=about 13 mm, D3 =about 7.3 mm, D4 =5.3 mm, D5=4.3 mm, D6=3.9 mm, D7=2.6 mm, t1=3.3 mm, t2=1.4 mm, t3=0.9 mm, and tA=1.15 mm.

[0120] The insulator 2 shown in FIG. 3B is designed to have slightlylarger outer diameters in its first and second front portions 2 g and 2i than in the example shown in FIG. 3A. It has, for example, thefollowing dimensions. L1=about 60 mm, L2=about 10 mm, L3=about 14 mm,D1=about 11 mm, D2=about 13 mm, D3=about 9.2 mm, D4=6.9 mm, D5=5.1 mm,D6=3.9 mm, D7=2.7 mm, t1=3.3 mm, t2=2.1 mm, t3=1.2 mm, and tA=1.65 mm.

[0121] As shown in FIG. 2, the glaze layer 2 d is formed on the outersurface of the insulator 2, more specifically, on the outer peripheralsurface of the rear portion 2 b. The glaze layer 2 d has a thickness of7 to 150 μm, preferably 10 to 50 μm. As shown in FIG. 1, the glaze layer2 d formed on the rear portion 2 b extends in the front directionfarther from the rear end of the metal shell 1 to a predeterminedlength, while the rear side extends till the rear end edge of the rearportion 2 b.

[0122] The glaze layer 2 d has the compositions explained in the columnsof the Means for solving the Problems, Works and Effects. As thecritical meaning in the composition range of each component has beenreferred to in detail, no repetition will be made herein. The thicknesstg (average value) of the glaze layer 2 d on the outer circumference ofthe base of the rear portion 2 b of the insulator (the cylindrical andouter circumference part projecting downward from the metal shell 1) is7 to 50 μm.

[0123] Now turning to FIG. 1, the ground electrode 4 and the core 3 a ofthe center electrode 3 are made of an Ni alloy and the like. The core 3a of the center electrode 3 is buried inside with a core material 3 bcomposed of Cu or Cu alloy or the like for accelerating heatdissipation. An ignition part 31 and an opposite ignition part 32 aremainly made of a noble metal alloy based on one kind or more of Ir, Ptand Rh. The core 3 a of the center electrode 3 is reduced in diameter ata front end and is formed to be flat at the front face, to which a diskmade of the alloy composing the ignition part is superposed, and theperiphery of the joint is welded by a laser welding, electron beamwelding, or resistance welding to form a welded part, therebyconstructing the ignition part 31. The opposite ignition part 32positions a tip to the ground electrode 4 at the position facing theignition part 31, and the periphery of the joint is welded to form asimilar welded part along an outer edge part. The tips are prepared by amolten metal comprising alloying components at a predetermined ratio orforming and sintering an alloy powder or a mixed powder of metals havinga predetermined ratio. At least one of the ignition part 31 and theopposite ignition part 32 may be omitted.

[0124] The spark plug 100 can be produced as follows. At first, as tothe insulator 2, an alumina powder is mixed with raw material powders ofa Si component, Ca component, Mg component, Ba component, and Bcomponent such that a predetermined mixing ratio is obtained in theabove mentioned composition in terms of oxides after sintering, and themixed powder is mixed with a predetermined amount of a binder (e.g.,PVA) and a water to prepare a slurry for forming the spark plug. The rawmaterial powders include, for example, SiO₂ powder as the Si component,CaCO₃ powder as the Ca component, MgO powder as the Mg component, BaCO₃or BaSO₄ as the Ba component, and H₃PO₃ as the B component. H₃BO₃ may beadded in the form of a solution.

[0125] A slurry is spray-dried into granules for forming a base, and thebase forming particles are rubber-pressed into a pressed body aprototype of the insulator. The formed body is processed on an outerside by grinding to the contour of the insulator 2 shown in FIG. 1, andthen baked 1400 to 1600° C. to obtain the insulator 2.

[0126] The glaze slurry is prepared as follows.

[0127] Raw material powders as sources of Si, B, Zn, Ba, alkalinecomponents (Na, K, Li), and raw powders of fluidity improving componentsare mixed for obtaining a predetermined composition. The F component isadded in a form of silicon fluoride high polymer or graphite fluoride.The mixed powder is heated and melted 1000 to 1500° C., and thrown intothe water to rapidly cool for vitrification, followed by grinding toprepare a glaze fritz. The glaze fritz is mixed with appropriate amountsof clay mineral, such as kaolin or gairome clay, and organic binder, andthe water is added thereto to prepare the glaze slurry.

[0128] The glaze slurry is sprayed from a nozzle to coat a requisitesurface of the insulator, thereby to form a coated layer of the glazeslurry as the glaze powder layer, and this is dried.

[0129] The center electrode 3 and the terminal metal fixture 13 arefitted in the insulator 2 formed with the glaze slurry coated layer, aswell as the resistor 15 and the electrically conductive glass seallayers 16, 17 are formed as follows. The center electrode 3 is insertedinto the first portion 6 a of the through hole 6. A conductive glasspowder is filled. The powder is preliminary compressed by pressing apress bar into the through hole 6 to form a first conductive glasspowder layer. A raw material powder for a resistor composition is filledand preliminary compressed in the same manner, so that the firstconductive glass powder, the resistor composition powder layer and asecond conductive glass powder layer are laminated from the centerelectrode 3 (lower side) into the through hole 6.

[0130] An assembled structure is formed where the terminal metal fixtureis disposed from the upper part into the through hole. The assembledstructure is put into a heating oven and heated at a predeterminedtemperature of 800 to 950° C. being above the glass dilatometricsoftening point, and then the terminal metal fixture 13 is pressed intothe through hole 6 from a side opposite to the center electrode 3 so asto press the superposed layers in the axial direction. Thereby, as seenin FIG. 1, the layers are each compressed and sintered to become aconductive glass seal layer 16, a resistor 15, and a conductive glassseal layer 17 (the above is the glass sealing step).

[0131] If the dilatometric softening point of the glaze powder containedin the glaze slurry coated layer is set to be 600 to 700° C., the glazeslurry coated layer can be baked at the same time as the heating in theabove mentioned glass sealing step, into the glaze layer 2 d. If theheating temperature of the glass sealing step is selected from therelatively low temperature as 800 to 950° C., oxidation to surfaces ofthe center electrode 3 and the terminal metal fixture 13 can be madeless to occur.

[0132] If a burner type-gas furnace is used as the heating oven (whichalso serves as the glaze baking oven),a heating atmosphere containsrelatively much steam as a combustion product. If the glaze compositioncontaining the B component of 40 mol % or less is used, the fluiditywhen baking the glaze can be secured even in such an atmosphere, and itis possible to form the glaze layer of smooth and homogeneous substanceand excellent in the insulation. The glaze-baking step can be in advanceperformed prior to the glass sealing step.

[0133] After the glass sealing step, the metal shell 1, the groundelectrode 4 and others are fitted on the structure to complete sparkplug 100 shown in FIG. 1. The spark plug 100 is screwed into an engineblock using the thread 7 thereof and used as a spark source to ignite anair/fuel mixture supplied to a combustion chamber. A high-tension cableor an ignition coil is interconnected to the spark plug 100 by means ofa rubber cap RC (composed of, e.g., silicone rubber) as shown with animaginary line in FIG. 1. The rubber cap RC has a smaller hole diameterthan the outer diameter D1 (FIG. 3) of the rear portion 2 b by about 0.5to 1.0 mm. The rear portion 2 b is pressed into the rubber cap whileelastically expanding the hole until it is covered therewith to itsbase. As a result, the rubber cap RC comes into close contact with theouter surface of the rear portion 2 b to function as an insulating coverfor preventing flashover.

[0134] By the way, the spark plug of the invention is not limited to thetype shown in FIG. 1, but, for example, the tip of the ground electrodeis made face the side of the center electrode to form an ignition gap.Further, a semi-planar discharge type spark plug is also useful wherethe front end of the insulator is advanced between the side of thecenter electrode and the front end of the ground electrode.

EXAMPLES

[0135] For confirmation of the effects according to the invention, thefollowing experiments were carried out.

Experimental Example 1

[0136] The insulator 2 was made as follows. Alumina powder (aluminacontent: 95 mol %; Na content (as Na₂O): 0.1 Mol %; average particlesize: 3.0 μm) was mixed at a predetermined mixing ratio with SiO₂(purity: 99.5%; average particle size: 1.5 μm), CaCO₃ (purity: 99.9%;average particle size: 2.0 μm), MgO (purity: 99.5%; average particlesize: 2 μm) BaCO₃ (purity: 99.5%; average particle size: 1.5 μm), H₃BO₃(purity: 99.0%; average particle size 1.5 μm), and ZnO (purity: 99.5%,average particle size: 2.0 μm). To 100 parts by weight of the resultingmixed powder were added 3 mass parts of PVA as a hydrophilic binder and103 mass parts of water, and the mixture was kneaded to prepare aslurry.

[0137] The resulting slurries with different compositions werespray-dried into spherical granules, which were sieved to obtainfraction of 50 to 100 μm. The granules were formed under a pressure of50 MPa by a known rubber-pressing method. The outer surface of theformed body was machined with the grinder into a predetermined figureand baked at 1550° C. to obtain the insulator 2. The X-ray fluorescenceanalysis revealed that the insulator 2 had the following composition.

[0138] Al component (as Al₂O₃): 94.9 mol %;

[0139] Si component (as SiO₂): 2.4 mol %;

[0140] Ca component (as CaO): 1.9 mol %;

[0141] Mg component (as MgO): 0.1 mol %;

[0142] Ba component (as BaO): 0.4 mol %; and

[0143] B component (as B₂O₃): 0.3 mol %.

[0144] The insulator 2 shown in FIG. 3A has the following dimensions.L1=about 60 mm, L2=about 8 mm, L3=about 14 mm, D1=about 10 mm, D2=about13 mm, D3=about 7 mm, D4=5.5, D5=4.5 mm, D6=4 mm, D7=2.6 mm, t1=1.5 mm,t2=1.45 mm, t3=1.25 mm, and tA=1.35 mm. In FIG. 1, a length LQ of theportion 2 k of the insulator 2 which projects over the rear end of themetal shell 1, is 25 mm.

[0145] Next, the glaze slurry was prepared as follows. SiO2 powder(purity: 99.5%), Al₂O₃powder (purity: 99.5%), H₃BO₃ powder (purity:98.5%), Na₂CO₃ powder (purity: 99.5%), K₂CO₃ powder (purity: 99%),Li₂CO₃ powder (purity: 99%), BaSO₄ powder (purity: 99.5%), SrCO₃ powder(purity: 99%), ZnO powder (purity: 99.5%), MoO₃ powder (purity: 99%),Fe₂O₃ powder (purity: 99%), WO₃ powder (purity: 99%), Ni₃O₄ powder(purity: 99%), Co₃O₄ powder (purity 99%), MnO₂ powder (purity: 99%), CaOpowder (purity: 99.5%), ZrO₂ powder (purity: 99.5%), TiO₂ powder(purity: 99.5%), MgO powder (purity: 99.5%), La₂O₃ powder (purity: 99%),Y₂O₃ powder (purity: 99.5%), Sc₂O₃ powder (purity: 99%), CeO₂ powder(purity: 99%), Pr₇O₁l powder (purity: 99%), Nd₂O₃ powder (purity: 99%),Sm₂O₃ powder (purity: 99%), Eu₂O₃ powder (purity: 99%), Gd₂O₃ powder(purity: 99%), Tb₂O₃ powder (purity: 99%), Dy₂O₃ powder (purity: 99%),Ho₂O₃ powder (purity: 99%), Er₂O₃ powder (purity: 99%), Tm₂03 powder(purity: 99%), Yb₂O₃ powder (purity: 99%), Lu₂O₃ powder (purity: 99%),Bi₂O₃ powder (purity: 99%), SnO₂ powder (purity: 99.5%), P₂O₅ powder(purity: 99%), Sb₂O₅ powder (purity: 99%), CuO powder (purity: 99%),Cr₂O₃ powder (purity: 99.5%), CaF₂ powder (purity: 98%), and LiF powder(purity: 98%) were mixed. The mixture was melted 1000 to 1500° C., andthe melt was poured into the water and rapidly cooled for vitrification,followed by grinding in an alumina pot mill to powder of 50 μm orsmaller. To 100 parts by weight of the glaze powder, 3 parts by weightof New Zealand kaolin and 2 parts by weight of PVA as an organic binderwere mixed, and the mixture was kneaded with 100 parts by weight of thewater to prepare the glaze slurry. F₂ is basically added as CaF₂, and ifall of Ca are added as CaF₂ but do not satisfy a predetermined value ofthe composition, it is supplemented by addition of LiF.

[0146] The glaze slurry was sprayed on the insulator 2 from the spraynozzle, and dried to form the coated layer of the glaze slurry having acoated thickness of about 100 μm. Several kinds of the spark plug 100shown in FIG. 1 were produced by using the insulator 2. The outerdiameter of the thread 7 was 14 mm. The resistor 15 was made of themixed powder consisting of B₂O₃—SiO₂—BaO—LiO₂ glass powder, ZrO₂powder,carbon black powder, TiO2powder, and metallic Al powder. Theelectrically conductive glass seal layers 16, 17 were made of the mixedpowder consisting of B₂O₃—SiO₂—Na₂O glass powder, Cu powder, Fe powder,and Fe—B powder. The heating temperature for the glass sealing, i.e.,the glaze baking temperature was set at 900° C.

[0147] On the other hand, the glaze which was not pulverized butsolidified into a mass was produced. It was confirmed that the massiveglaze was vitrified (amorphous) by the X-ray diffraction, and themassive glaze was performed with the following experiment.

[0148] {circle over (1)} Analysis of the chemical composition: By thefluorescent X-ray analysis. Analyzed values of the respective samples(in terms of oxide) are shown in Tables 1 to 6. The compositions of theglaze layer 2 d formed on the surface of the insulator 2 were measuredby the EPMA method, and it was confirmed that the measured resultsalmost met the analyzed values measured by use of the massive samples.

[0149] {circle over (2)} Thermal expansion coefficient: The specimen of5 mm×5 mm×5 mm was cut out from the block-like sample, and measured withthe known dilatometer method at the temperature ranging 20 to 350° C.The same measurement was made at the same size of the specimen cut outfrom the insulator 2. As a result, the value was 73×10⁻⁷/° C.

[0150] {circle over (3)} Dilatometric softening point: The powder sampleweighing 50 mg was subjected to the differential thermal analysis, andthe heating was measured from a room temperature. The second endothermicpeal was taken as the dilatometric softening point. TABLE 1 1 2* 3* 4 56 7 8 SiO₂ 42.0 44.0 49.0 42.0 41.0 33.0 40.0 40.0 Al₂O₃ 1.5 1.5 1.5 0.51.5 0.5 1.5 1.5 B₂O₃ 31.0 31.0 31.0 34.0 31.0 23.0 30.0 30.0 Na₂O 1.02.0 1.0 2.0 2.0 1.0 1.0 K₂O 4.0 5.0 4.0 4.0 3.0 3.0 Li₂O 3.0 6.0 2.0 4.02.0 2.0 BaO 4.5 4.5 4.5 4.5 3.0 5.0 5.0 SrO 4.5 2.0 ZnO 8.0 8.0 5.5 8.09.0 9.0 9.0 9.0 MoO₃ 1.0 1.0 1.0 FeO 1.0 WO₃ Ni₃O₄ Co₃O₄ MnO₂ CaO 2.01.0 10.0 2.0 2.0 ZrO₂ 1.0 1.0 1.0 1.0 TiO₂ 1.5 1.5 HfO₂ 1.5 1.5 MgO F₂4.0 4.0 0.5 0.5 3.0 9.0 2.0 2.5 La₂O₃ 1.0 Y₂O₃ Sc₂O₃ Pr₇O₁₁ Sm₂O₃ Eu₂O₃Gd₂O₃ Tb₂O₃ Dy₂O₃ Ho₂O₃ Er₂O₃ Tm₂O₃ Yb₂O₃ Lu₂O₃ Bi₂O₃ 4.0 1.0 0.5 SnO₂P₂O₅ Sb₂O₅ CuO CeO₂ Cr₂O₃ Total 100 100 100 100 100 100 100 100

[0151] TABLE 2 9 10 11 12 13 14 15 16 SiO₂ 40.0 38.0 39.0 39.0 40.0 40.040.0 40.0 Al₂O₃ 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 B₂O₃ 30.0 30.0 30.0 30.030.0 30.0 30.0 30.0 Na₂O 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 K₂O 3.0 3.0 3.03.0 3.0 3.0 3.0 3.0 Li₂O 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 BaO 5.0 5.0 5.05.0 5.0 5.0 5.0 5.0 SrO ZnO 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 MoO₃ 1.0 1.01.0 1.0 FeO WO₃ 1.0 Ni₃O₄ 1.0 Co₃O₄ 1.0 MnO₂ 1.0 CaO 2.0 1.0 1.0 1.0 2.02.0 2.0 2.0 ZrO₂ 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 TiO₂ 1.5 1.5 1.5 1.51.5 1.5 1.5 1.5 HfO₂ MgO 2.0 2.0 2.0 F₂ 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0La₂O₃ 0.5 1.0 Y₂O₃ 1.0 Sc₂O₃ 1.0 Pr₇O₁₁ 1.0 Sm₂O₃ Eu₂O₃ Gd₂O₃ Tb₂O₃Dy₂O₃ Ho₂O₃ Er₂O₃ Tm₂O₃ Yb₂O₃ Lu₂O₃ Bi₂O₃ 0.5 2.0 1.0 1.0 SnO₂ P₂O₅Sb₂O₅ CuO CeO₂ Cr₂O₃ Total 100 100 100 100 100 100 100 100

[0152] TABLE 3 17 18 19 20 21 22 23 24 SiO₂ 40.0 40.0 40.0 40.0 40.040.0 40.0 40.0 Al₂O₃ 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 B₂O₃ 30.0 30.0 30.030.0 30.0 30.0 30.0 30.0 Na₂O 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 K₂O 3.03.0 3.0 3.0 3.0 3.0 3.0 3.0 Li₂O 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 BaO 5.05.0 5.0 5.0 5.0 5.0 5.0 5.0 SrO ZnO 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 MoO₃1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 FeO WO₃ Ni₃O₄ Co₃O₄ MnO₂ CaO 2.0 2.0 2.02.0 2.0 2.0 2.0 2.0 ZrO₂ 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 TiO₂ 1.5 1.51.5 1.5 1.5 1.5 1.5 1.5 HfO₂ MgO F₂ 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0La₂O₃ Y₂O₃ Sc₂O₃ Pr₇O₁₁ Sm₂O₃ 1.0 Eu₂O₃ 1.0 Gd₂O₃ 1.0 Tb₂O₃ 1.0 Dy₂O₃1.0 Ho₂O₃ 1.0 Er₂O₃ 1.0 Tm₂O₃ 1.0 Yb₂O₃ Lu₂O₃ Bi₂O₃ SnO₂ P₂O₅ Sb₂O₅ CuOCeO₂ Cr₂O₃ Total 100 100 100 100 100 100 100 100

[0153] TABLE 4 25 26 27 28 29 30 31 32 SiO₂ 40.0 40.0 40.0 40.0 40.040.0 40.0 40.0 Al₂O₃ 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 B₂O₃ 30.0 30.0 30.030.0 30.0 30.0 30.0 30.0 Na₂O 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 K₂O 3.03.0 3.0 3.0 3.0 3.0 3.0 3.0 Li₂O 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 BaO 5.05.0 5.0 5.0 5.0 5.0 5.0 5.0 SrO ZnO 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 MoO₃1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 FeO WO₃ Ni₃O₄ Co₃O₄ MnO₂ CaO 2.0 2.0 2.02.0 2.0 2.0 2.0 2.0 ZrO₂ 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 TiO₂ 1.5 1.50.5 0.5 0.5 0.5 0.5 0.5 HfO₂ MgO F₂ 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0La₂O₃ Y₂O₃ Sc₂O₃ Pr₇O₁₁ Sm₂O₃ Eu₂O₃ Gd₂O₃ Tb₂O₃ Dy₂O₃ Ho₂O₃ Er₂O₃ Tm₂O₃Yb₂O₃ 1.0 Lu₂O₃ 1.0 Bi₂O₃ 1.0 1.0 1.0 1.0 1.0 1.0 SnO₂ 1.0 P₂O₅ 1.0Sb₂O₅ 1.0 CuO 1.0 CeO₂ 1.0 Cr₂O₃ 1.0 Total 100 100 100 100 100 100 100100

[0154] TABLE 5 33* 34* 35* 36* 37* 38* 39* 40* SiO₂ 28.0 61.0 49.0 31.043.0 35.0 42.0 35.0 Al₂O₃ 1.5 0.5 1.5 0.5 1.5 0.5 1.5 1.5 B₂O ₃ 40.021.0 18.0 51.0 35.0 22.0 35.0 28.0 Na₂O 1.0 0.5 1.0 1.0 1.0 1.0 1.0 1.0K₂O 3.0 3.0 3.0 3.5 4.5 4.0 4.5 4.0 Li₂O 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0BaO 6.0 3.0 6.0 3.0 7.5 3.0 16.0 SrO 1.0 2.0 ZnO 9.0 5.0 9.0 5.0 26.58.0 3.0 MoO₃ 2.0 1.0 2.0 FeO WO₃ Ni₃O₄ Co₃O₄ MnO₂ CaO 2.0 2.0 2.0 2.02.0 2.0 ZrO₂ 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 TiO₂ 1.5 1.5 1.5 1.5 1.5HfO₂ MgO F₂ 2.0 2.0 2.0 1.0 1.0 2.0 0.5 2.0 La₂O₃ Y₂O₃ Sc₂O₃ Pr₇O₁₁Sm₂O₃ Eu₂O₃ Gd₂O₃ Tb₂O₃ Dy₂O₃ Ho₂O₃ Er₂O₃ Tm₂O₃ Yb₂O₃ Lu₂O₃ Bi₂O₃ 1.01.0 1.0 1.0 1.0 1.0 SnO₂ P₂O₅ Sb₂O₅ CuO CeO₂ Cr₂O₃ Total 100 100 100 100100 100 100 100

[0155] TABLE 6 41* 42* 43* 44* 45* 46 SiO₂ 45.0 37.0 54.0 55.0 32.0 37.0Al₂O₃ 1.5 1.0 1.5 1.5 0.5 1.5 B₂O₃ 31.5 30.0 22.0 22.0 23.0 30.0 Na₂O4.0 1.0 1.0 1.0 1.0 K₂O 1.0 2.0 3.0 3.0 3.0 3.0 Li₂O 0.5 7.0 2.0 3.0 2.0BaO 5.0 5.0 5.0 5.0 5.0 5.0 SrO ZnO 8.0 8.0 7.0 7.0 9.0 9.0 MoO₃ 1.0 1.01.0 1.0 FeO WO₃ Ni₂O₄ Co₃O₄ MnO₂ CaO 2.0 1.5 2.0 2.0 9.0 2.0 ZrO₂ 1.01.0 1.0 1.0 1.0 1.0 TiO₂ 1.5 0.5 0.5 0.5 0.5 0.5 HfO₂ MgO 0.5 F₂ 2.0 2.011.0 1.0 La₂O₃ Y₂O₃ Sc₂O₃ Pr₇O₁₁ Sm₂O₃ Eu₂O₃ Gd₂O₃ Tb₂O₃ Dy₂O₃ Ho₂O₃Er₂O₃ Tm₂O₃ Yb₂O₃ Lu₂O₃ Bi₂O₃ 1.0 1.0 1.0 6.0 SnO₂ P₂O₅ Sb₂O₅ CuO CeO₂Cr₂O₃ Total 100 100 100 100 100 100

[0156] With respect to the respective spark plugs, the insulationresistance at 500° C. was evaluated at the applied voltage 1000V throughthe already explained process. Further, the outer appearance of theglaze layer 2 d formed on the insulator 2 was visually observed. Thefilm thickness of the glaze layer on the outer circumference of the baseedge part of the insulator was measured in the cross section by the SEMobservation. With respect to judgements on the outer appearances of theglaze layers, the outer appearances of brilliance and transparencywithout abnormality are excellent (∘), those within a permissive rangebut recognized with crimpings and devitrifications are good (Δ), andthose with apparent abnormality are shown with kinds of abnormalities inmargins. The abovementioned results are shown in Tables 7 to 11. TABLE 71 2* 3* 4 5 6 7 8 9 10 F₂/R₂O 0.57 0.57 0.07 0.07 0.50 1.50 0.33 0.4230.33 0.33 ZnO + BaO 12.5 12.5 10.0 12.5 13.5 14.0 14.0 14.0 14.0 14.0Al₂O₃ + 3.5 1.5 2.5 0.5 1.5 10.5 3.5 3.5 3.5 4.5 CaO + MgO K₂O/R₂O 0.60.0 0.7 0.6 0.0 0.7 0.5 0.5 0.5 0.5 Li₂O/R₂O 0.4 0.9 0.0 0.3 0.7 0.0 0.30.3 0.3 0.3 Thermal 6.90 6.20 7.10 6.70 6.30 6.80 6.60 6.60 6.60 6.65expansion coefficient × 10⁻⁶ Dilato- 530 520 600 565 510 500 550 555 550540 metric softening point 500° C. 1200 MΩ 300 1500 1400 600 1600 15001500 1500 1600 insulating resistance External ◯ ◯ X ◯ ◯ ◯ ◯ ◯ ◯ ◯appearance (Glaze crimping) Special remark

[0157] TABLE 8 11 12 13 14 15 16 17 18 19 20 F₂/R₂O 0.33 0.33 0.33 0.330.33 0.33 0.33 0.33 0.33 0.33 ZnO + BaO 14.0 14.0 14.0 14.0 14.0 14.014.0 14.0 14.0 14.0 Al₂O₃ + 4.5 4.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5CaO + MgO K₂O/R₂O 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Li₂O/R₂O 0.30.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Thermal 6.65 6.65 6.60 6.60 6.606.60 6.60 6.60 6.60 6.60 expansion coefficient × 10⁻⁶ Dilato- 550 550555 555 555 555 555 555 555 555 metric softening point 500° C. 1600 16001500 1500 1500 1500 1500 1500 1500 1500 insulating resistance External ◯◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ appearance Special remark

[0158] TABLE 9 21 22 23 24 25 26 27 28 29 30 F₂/R₂O 0.33 0.33 0.33 0.330.33 0.33 0.33 0.33 0.33 0.33 ZnO + BaO 14.0 14.0 14.0 14.0 14.0 14.014.0 14.0 14.0 14.0 Al₂O₃ + 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5CaO + MgO K₂O/R₂O 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Li₂O/R₂O 0.30.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Thermal 6.60 6.60 6.60 6.60 6.606.60 6.60 6.60 6.60 6.60 expansion coefficient × 10⁻⁶ Dilato- 555 555555 555 555 555 550 550 550 550 metric softening point 500° C. 1500 15001500 1500 1500 1500 1500 1500 1500 1500 insulating resistance External ◯◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ appearance Special remark

[0159] TABLE 10 31 32 33* 34* 35* 36* 37* 38* 39* 40* F₂/R₂O 0.33 0.330.33 0.36 0.33 0.15 0.13 0.29 0.07 0.29 ZnO + BaO 14.0 14.0 15.0 8.016.0 8.0 7.5 29.5 8.0 21.0 Al₂O₃ + 3.5 3.5 3.5 0.5 3.5 0.5 3.5 2.5 3.53.5 CaO + MgO K₂O/R₂O 0.5 0.5 0.5 0.5 0.5 0.5 0.6 0.6 0.6 0.6 Li₂O/R₂O0.3 0.3 0.3 0.4 0.3 0.3 0.3 0.3 0.3 0.3 Thermal 6.60 6.60 6.75 6.20 6.806.30 6.90 6.40 6.50 7.40 expansion coefficient × 10⁻⁶ Dilato- 550 550490 630 615 495 600 505 595 505 metric softening point 500° C. 1500 15001000 1700 1600 1400 1700 600 1600 1700 insulating resistance External ◯◯ X X X X X X X X appearance (Crimping) (A) (A) (Crimping) (A) (B) (A)(Crazing) Special Water Water Thermal remark proof: proof: expansion:Bad: Bad: Large

[0160] TABLE 11 41* 42* 43* 44* 45* 46 F₂/R₂O 1.33 0.15 0.00 0.00 1.570.17 ZnO + BaO 13.0 13.0 12.0 12.0 14.0 14.0 Al₂O₃+ 3.5 2.5 3.5 3.5 10.03.5 CaO + MgO K₂O/R₂O 0.7 0.2 0.5 0.8 0.4 0.5 Li₂O/R₂O 0.3 0.5 0.3 0.00.4 0.3 Thermal 6.25 7.70 6.45 6.45 6.55 6.55 expansion coefficient ×10⁻⁶ Dilatometric 620 500 600 590 510 490 softening point 500° C. 2000150 1600 1600 1500 1400 insulating resistance External X ◯ X X X Δappearance (A) (A) (A) (B) (Coloring) Special remark

[0161] According to the results, depending on the compositions of theglaze of the invention, although no Pb is substantially contained, theglaze may be baked at relatively low temperatures, sufficient insulatingproperties are secured, and the outer appearance of the baked glazefaces are almost satisfied.

[0162] This application is based on Japanese Patent application JP2001-192668, filed Jun. 26, 2001, the entire content of which is herebyincorporated by reference, the same as if set forth at length.

What is claimed is:
 1. A spark plug comprising: a center electrode; ametal shell; and an alumina ceramic insulator disposed between thecenter electrode and the metal shell, wherein at least part of thesurface of the insulator is covered with a glaze layer comprisingoxides, wherein the glaze layer comprises: 1 mol % or less of a Pbcomponent in terms of PbO; to 60 mol % of a Si component in terms ofSiO₂; to 50 mol % of a B component in terms of B₂O₃; 0.5 to 25 mol % ofa Zn component in terms of ZnO; 0.5 to 15 mol % in total of at least oneof Ba and Sr components in terms of BaO and SrO, respectively; 2 to 12mol % in total of at least two alkaline metal components of Na, K andLi, in terms of Na₂O, K₂O, and Li₂O, respectively, wherein K and Li isessential; and 0.1 to 10 mol % of a F component in terms of F₂.
 2. Aspark plug comprising: a center electrode; a metal shell; and an aluminaceramic insulator disposed between the center electrode and the metalshell, wherein at least part of the surface of the insulator is coveredwith a glaze layer comprising oxides, wherein the glaze layer comprises:1 mol % or less of a Pb component in terms of PbO; to 60 mol % of a Sicomponent in terms of SiO₂; to 40 mol % of a B component in terms ofB₂O₃; 0.5 to 25 mol % of a Zn component in terms of ZnO; 0.5 to 15 mol %in total of at least one of Ba and Sr components in terms of BaO andSrO, respectively; 2 to 12 mol % in total of at least one alkaline metalcomponent of Na, K and Li, in terms of Na₂O, K₂O, and Li₂O,respectively; 0.1 to 10 mol % of a F component in terms of F₂; 0.1 to 5mol % in total of at least one component of Bi, Sb and rare earth RE, REbeing at least one selected from Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb,Dy, Ho, Er, Tm, Yb and Lu, in terms of Bi₂O₃, Sb₂O₅ and RE₂O₃,respectively, proviso that Ce is in terms of CeO₂ and Pr is in terms ofPr₇O₁₁.
 3. The spark plug according to claim 1, wherein the glaze layercomprises 7 to 25 mol % in total of the Zn component and the at leastone of Ba and Sr components in terms of ZnO, BaO and SrO, respectively.4. The spark plug according to claim 1, wherein the glaze layercomprises NR mol % in total of the at least two alkaline metalcomponents and NF mol % of the F component, and the glaze layersatisfies a relationship: NF/NR is from 0.07 to 1.5.
 5. The spark plugaccording to claim 1, wherein the glaze later further comprises 0.5 to 5mol % in total of at least one of Mo, W, Ni, Co, Fe and Mn components interms of MoO₃, WO₃, Ni₃O₄, Co₃O₄, Fe₂O₃, and MnO₂, respectively.
 6. Thespark plug according to claim 1, wherein the glaze layer furthercomprises 0.5 to 5 mol % in total of at least one of Zr, Ti and Hfcomponents in terms of ZrO₂, TiO₂ and HfO₂, respectively.
 7. The sparkplug according to claim 1, wherein the glaze layer further comprises 0.5to 15 mol % in total of at least one of 0.5 to 5 mol % of an Alcomponent in terms of Al₂O₃, 0.5 to 10 mol % of a Ca component in termsof CaO, and 0.5 to 10 mol % of a Mg component in terms of MgO.
 8. Thespark plug according to claim 1, wherein the glaze layer furthercomprises 0.5 to 5 mol % in total of at least one of Sn, P, Cu and Crcomponents in terms of SnO₂, P₂O₅, CuO and Cr₂O₃, respectively.
 9. Thespark plug according to claim 1, which comprises one of: a terminalmetal fixture and the center electrode as one body, in a through hole ofthe insulator; and a terminal metal fixture provided separately from thecenter electrode via a conductive bonding layer, and an insulationresistant value is 400 MΩ or more, which is measured by keeping thewhole of the spark plug at about 500° C. and passing a current betweenthe terminal metal fixture and the metal shell via the insulator. 10.The spark plug according to claim 1, wherein the insulator comprises analumina insulating material comprising 85 to 98 mol % of an Al componentin terms of Al₂O₃, and the glaze layer has an average thermal expansioncoefficient at the temperature ranging 20 to 350° C. of 5×10⁻⁶/° C. to8.5×10⁻⁶/° C.
 11. The sparkplug according to claim 1,whereintheglazelayer has a dilatometric softening point of 520 to 620° C.